The present invention relates to a display correction system for a display device, in particular, a display device having a nonvolatile memory device and a display device correction method.
As a conventional display device, a cathode ray tube (CRT) device, a plasma display device, a liquid crystal display (LCD) device or the like is used. Recently, an LCD device, in particular, an active matrix type LCD device has been developed.
Since a dielectric constant of a liquid crystal substance (molecule) in a parallel direction to a molecule axis is different from that in a vertical direction, an LCD device uses this characteristic and displays ON/OFF states, that is, light/dark states by adjusting a transmission quantity or a scattering quantity of a light. In general, a liquid crystal material includes, a twisted nematic (TN) liquid crystal, a super twisted nematic (STN) liquid crystal, a ferroelectric liquid crystal and the like.
It is said that among display devices using a liquid crystal, an active matrix type LCD device has a superior display characteristic. As shown in FIG. 2, in a conventional active matrix type LCD device, source lines 203 to 205 connected to a source driver 201 are combined with gate lines 206 to 208 connected to a gate driver 202 in a matrix form, and thin film transistors (TFTs) 209 to 212 are arranged in these intersection portions. Gate electrodes in TFTs are connected to the gate lines 206 to 208, source electrodes in the TFTs are connected to the source lines 203 to 205, and drain electrodes in the TFTs are connected to pixel electrodes and retaining capacitors 213 to 216. Liquid crystal 217 to 220 are arranged between the pixel electrodes.
FIGS. 3A to 3C show operation waveforms of the TFT. When signal voltages are applied to the gate electrode and the source electrode of the TFT, the TFT is turned on, so that a voltage on the pixel electrode almost coincides with a source voltage. When the signal voltage is not applied to the gate electrode of the TFT, the TFT is turned off, so that a voltage on the pixel electrode is maintained until the TFT is turned on next.
Since a voltage is applied from the pixel electrode to the liquid crystal by the above manner, an LCD device having less crosstalk and a large contrast with respect to adjacent pixels can be produced.
In an active matrix type LCD device described above, since the necessary number of TFTs corresponds to the number of pixels, a defect produces in a TFT element formed in a substrate. If a TFT having a defect is in an open state or a short circuit state, a defect pixel has a desired voltage or a voltage on the defect pixel is unstable, so that the pixel produces as a point defect on a panel. Also, by variations of a threshold value and a mobility in a TFT, a voltage applied to a pixel electrode is varied, so that variations in brightness (gradation) of pixels produce.
To solve the above problem, as shown in FIG. 4, in a pixel portion having gate lines 401 and 402 and source lines 403 and 404, a plurality of TFTs 405 and 406 are arranged for one pixel electrode 407 to obtain redundancy. That is, if the TFT 406 is a defective element, a drain terminal is scribed (cut) in a laser scribing portion 408 using a laser or the like to remove a defective element.
A defective element is detected by using a structure as shown in FIG. 5. In FIG. 5, the structure includes gate lines 501 and 502, source lines 503 and 504, common electrode line 505, TFTs 506 to 509, retaining capacitors 510 and 511, switches 512 to 515, amplifiers 516 and 517, measurement terminals 518 and 519 and power sources 520 and 521. A voltage by which a TFT is turned on sufficiently is applied to the gate lines 501 and 502 connected to elements to be examined through the switches 512 and 513 and simultaneously a desired voltage is applied to the source lines 503 and 504 connected to the elements through the switches 514 and 515. Next, a voltage on the gate lines is set to a ground level, to turn off the TFTs.
Application of the voltage to the source lines is stopped and then the TFTs are leaved for a desired period of time. After that, a voltage is applied to the gate lines again and then a voltage on the source lines is measured. When the TFTs are in a normal state, since a first source voltage is maintained by the retaining capacitors, it is measured. Also, When a drain and a source in each TFT are in a short circuit state, since discharge produces in each TFT through resistors connected to the source lines while leaving the TFTs, a voltage is varied in measurement. Further, when the TFTs are in an open state, even though a voltage is applied to the gate lines, the voltage is not applied to the retaining capacitors. Furthermore, even through the retaining capacitors are charged insufficiently by variations of mobility and threshold of the TFT, a defect element can be distinguished by voltage measurement with high accuracy.
In a conventional LCD device and a correction method for the device, as described above, the following problems produce. Though a defect of a pixel TFT can be corrected, a nonuniform display due to variation of a liquid crystal material and a band nonuniformity produced by rubbing cannot be corrected, so that an LCD device becomes a defective product in many cases.
In general, when an operator watches a display device, two pixels which are sufficiently apart from each other cannot be distinguished even though a brightness (gradation) difference between one pixel and the other pixel is 10% or more. However, adjacent pixels are distinguished each other even though the brightness difference is about 2%. Therefore, it is important to suppress variations of brightness between adjacent pixels.
An LCD device has a tendency to extend a size of a display portion (screen portion). In accordance with an increase of the size, it is difficult to maintain uniform display, so that a yield of an LCD device is deteriorated. Also, in a personal computer and a work station, since an operator continues to watch a display portion at a near position for a long period of time, nonuniform display is disagreeable for an operator and efficiency of the operator decreases, so that it is a claim subject from a user.